Abstract: Late Oxfordian-Barremian Basin Filling Systems of the West Siberian Basin

Vladimir N. Vyssotski, William H. Kanes, Jerome A. Eyer, Alan Nairn

The West Siberian basin is the largest epicontinental basin in the world. The three major Stages can be recognized in the Mesozoic-Cenozoic depositional history: Rift-Related, Basin-Wide, and Uplift-Related. The Upper Oxfordian-Barremian age sequence is a large transgressive/regressive wedge in the West Siberian sedimentary basin and covers 2.3 million sq km and ranges in thickness from 200-1190 m. It is the second supersequence in the Basin-Wide Stage of the West Siberian basin. More than 1130 oil and gas pools in 246 fields are present in this stratigraphic interval.

The basal transgression was formed in the environmental conditions from existing Upper Oxfordian to Valanginian. Well pronounced Late Oxfordian-Kimmeridgian basal transgressive system tracts occur within central the Kaimysov-Middle Ob-Yamal area. There are lagoonal clays, shallow marine sands and muds, and distal dark-gray clays. The east and west basin margins were developed as separate basins with progradational filling and had merged into the West Siberian basin during Kimmeridgian and Berriasian respectively. Both basal transgressive and marginal prograding sequences include hydrocarbons.

The progradational complex has an asymmetric structure within the basin with the final depocenter in the western Frolov depression. Westwards a prograding complex 1200 m thick covers about 85% of the basin and appears to be the major oil-bearing sequence. Sandstone content decreases basinward. Within the westward prograding complex three basic drainage systems can be distinguished: (1) silty-sand Nadym-Pur-Taz, clay, (2) sandy-clay Kaimysov-Middle Ob-Paydugin, and (3) the predominantly sandy Yenisey-Khatanga-Yamal. They contain traps of a variety of geneses. There are three main levels of sandstone distribution in the marine portion of the westward complexes: laterally persistence shelf, shelf edge detached shingled fans, and slope/basin floor fans. They are principal hydrocarbon bear ng reservoirs. In addition, bituminous distal clays are oil bearing. Coastal plain and fluvial deposits of different basin filling systems are distinguished by sand rate and number of oil fields as well. The coastal plain lithofacies of the drainage area 2 is composed of clay deposits with rare and laterally discontinuous lower delta plain sand. Within area 1 and 3 they consist of primarily sand and distribution of traps is controlled by clay layers. The fluvial lithofacies is comprised of clay and clayey-sand with widespread, laterally discontinuous, incised valley sand (western part) in the area 2 and by sand and sand-clay intercalation (western part) within areas 1 and 3. Therefore, they are usually oil saturated in the top of the sequence, overlain by Aptian transgressive clay.

The northward prograding complex of the South Priural area is composed of clay and silty clay sediments 150-400 m thick. Eastward prograding sequence of the Priural area consists of 200-500 m of predominantly clay, silt and silty clay deposits.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994